We propose a two-phase prospective study of drug-related injuries, which we call adverse drug events (ADEs), to identify their causes and to evaluate strategies for their prevention. The study will be a collaborative effort between the Harvard School of Public Health, the Brigham and Women's Hospital, and the Massachusetts General Hospital. In Phase 1, the subject of this proposal, nurse-investigators will identify ADEs and potential ADEs (PADEs) in hospitalized patients by reviewing charts and interviewing hospital personnel. We will classify ADEs and PADEs by severity, preventability, service responsible, and attribution. In addition, we will identify patient risk factors for and resource utilization associated with ADEs. From these data we will construct a complete profile of drug-related injury, including types of injuries, risk factors, and costs. We will investigate the causes of preventable ADEs and PADEs by interviewing hospital personnel. We will analyze this information to identify systems failures responsible for the injuries and possible changes. We will present the results of these investigations to senior level steering committees at each hospital. The steering committees will select problem areas for improvement and will appoint multidisciplinary unit-based quality improvement teams at each hospital to design and implement systems changes to reduce the identified ADEs and PADEs. In Phase II, for which we will submit a separate proposal based on our findings in Phase 1, we will evaluate the effectiveness of these systems changes in a randomized, controlled trial, using the same units as in Phase I to also allow time series comparisons.